Fix ERC20DividendableEth for transfers.

[alcueca]

Fix the issues with the supply first, please.

A mapping(address => uint) public dividendsPerTokenAdjustment is required to keep track of imbalances when transferring tokens with owning dividends.

In a transfer:
transferWeight = trasnferAmount / recipientBalance (Fixed point)
dividendsPerTokenDifferential = lastDividendsPerToken[sender]-lastDividendsPerToken[receiver]
dividendsPerTokenAdjustment[account] = dividendsPerTokenAdjustment[account] + dividendsPerTokenDifferential * transferWeight

When calculating dividends owed:
owedDividendsPerToken = dividendsPerToken - lastDividendsPerToken[account] + dividendsPerTokenAdjustment[account]

When claiming dividends:
dividendsPerTokenAdjustment[account] = 0

[alcueca]

You can use these two test cases:

Test case 1: Transfers of dividend carrying tokens also transfer the dividends.
Supply: 100 tokens, User 1 has 40, User 2 has 60.
Distribute 10 ether.
User 2 claims dividends. He's got 6 ether now.
User 1 transfers 6 tokens to user 2.
User 2 claims dividends again. He's got 6.6 ether now.

Test case 2: Transfers of non dividend carrying tokens don't transfer any dividends.
Supply: 100 tokens, User 1 has 40, User 2 has 60.
Distribute 10 ether.
User 1 claims dividends.
User 1 transfers 6 tokens to user 2.
User 2 claims dividends again. He's got 6 ether now.